(a) Field of the Invention
The present invention relates to a real image finder and, more particularly, to a real image finder which converts an inverted image formed through an objective lens group into an erect image by using two prisms and provides good image quality and a wide viewing angle.
(b) Description of the Related Art
In order to meet the current trend of being compact and light-weight, a camera must have a compact finder with a high zoom ratio as well as a small photographing system.
A finder used in a photographing system generally comprises an objective lens group and an eyepiece lens group. An image of an object is formed through the objective lens group and is magnified by the eyepiece lens group, so that a user can observe the image of the object through the eyepiece lens group. However, without an inverting eyepiece lens group, a user observes an inverted image because the objective lens group inverts the image upside down.
Consequently, a system for erecting the inverted image formed through an objective lens group is necessary either in an eyepiece lens group or an objective lens group.
In order to make a camera as compact as possible, methods for using a mirror or a prism to erect an inverted image have been proposed. Such methods have been disclosed in Japanese patent laid-open No. Hei 6-95215 and Japanese patent laid-open No. Hei 5-297274.
Japanese patent laid-open No. Hei 6-95215 proposes a finder which comprises an objective lens group, an eyepiece lens group, and two prisms. The prisms are located between the objective lens group and the eyepiece lens group, and serve to invert an image. Although the disclosure of the Japanese patent laid-open No. 6-95215 achieves a compact finder by reducing the total length of the system, the viewing angle at a wide-angle position is only 50.degree. and the zoom ratio is a mere 1.75, both of which are inadequate.
Japanese patent laid-open No. Hei 5-297274 does not clearly describe how to erect an inverted image. Consequently, a longer focal length becomes necessary to achieve an erecting system. Thus, the resultant magnification ratio is low and a viewing angle becomes as small as 1.5 degrees, both of which are inadequate.
Furthermore, the disclosure in the Japanese patent laid-open No. Hei 5-297274 allows such a short eye relief distance as 15 mm that it is difficult to achieve a finder convenient for observation.